This invention relates to an exposure apparatus for use in a semiconductor device manufacturing process and, more particularly, to a scanning exposure apparatus wherein, in projection exposure for printing a pattern of a photomask or reticle onto a wafer, the mask and the wafer are scanned in synchronism with each other relative to a projection optical system. In another aspect, the invention is concerned with a device manufacturing method which uses such a scanning exposure apparatus.
Semiconductor manufacturing technology is being advanced considerably, and fine processing techniques therefor are also developing more and more. In optical processing techniques, reduction projection exposure apparatuses with submicron order resolution, called steppers, are used prevalently. Further improvement of resolution, further enlargement of numerical aperture (NA), and a further reduction of the wavelength of exposure light are desired in this field.
Those scanning exposure apparatus have received much attraction, such as one wherein a unit-magnification scanning exposure apparatus having a reflection projection optical system is modified and a refractive element or elements are incorporated into the projection exposure apparatus, or one wherein a reduction projection optical system consisting only of refractive elements is used and both a mask stage and a stage (wafer stage) for a photosensitive substrate are scanned in synchronism with each other and at a speed ratio corresponding to the reduction magnification.
For mask pattern image focusing in these types of exposure apparatuses, level measurement and autofocus and autoleveing correction drive are successively carried out during the scan exposure, so as to sequentially register the surface of a photosensitive substrate to be exposed with respect to the best imaging plane of a projection optical system.
For the level and surface position detecting mechanism to be used in these types of apparatuses, there is a method wherein an oblique incidence optical system in which light is projected to a wafer surface obliquely from above is used and wherein reflection light from the photosensitive substrate is detected as a positional deviation upon a sensor, or a method wherein a gap sensor such as an air micro-sensor or an electrostatic capacitance sensor is used. In these methods, from level measured values obtained during the scan, correction drive for the level and tilt of each measurement position as it passes the exposure slit region are calculated, and then correction is carried out.
Conventionally, in step-and-repeat type exposure apparatuses or scanning exposure apparatuses as described above, the focusing mechanism for the photosensitive substrate and the imaging plane of the projection optical system are provided only on the photosensitive substrate side.
In scanning exposure apparatuses, however, there are cases wherein the surface for carrying a mask thereon tilts with scan of the mask, relative to a mask stage. If this occurs, it causes a change in tilt of the mask which cannot be corrected by the focusing mechanism upon the exposure surface of the photosensitive substrate. Thus, it results in defocus.
For example, if a six-inch mask is used and the surface for carrying the mask thereon tilts by 20 ppm with respect to the scan direction, then the distance between the mask pattern and a projection optical system may change by about 3 microns during the scan of the whole mask surface. If the projection optical system has a reduction magnification of 1:4, the amount of shift of the image plane may be 0.19 micron. The numerical aperture of projection optical systems is increasing so as to meet the miniaturization of a circuit pattern, and the allowable depth of focus during the transfer process is decreasing. Thus, such a shift cannot be disregarded.
It is accordingly an object of the present invention to provide a scanning exposure apparatus and/or a device manufacturing method by which, even if tilt of a mask carrying surface with respect to a mask stage changes, the surface of a photosensitive substrate to be exposed can be focused precisely with respect to an imaging plane of a projection optical system, constantly.
In accordance with an aspect of the present invention, there is provided a scanning exposure apparatus, comprising: a first movable stage being movable while carrying a first object (mask) thereon; a second movable stage being movable while carrying a second object (wafer) thereon; a projection optical system; scanning means for scanningly moving said first and second stages in synchronism with each other and relative to said projection optical system so as to project a pattern of the first object onto the second object; reference plates provided on said first movable stage and having predetermined patterns; detecting means for detecting positions of said reference plates with respect to an exposure optical axis direction of said projection optical system; measuring means for measuring a position of said first movable stage with respect to a scan direction; storing means for storing therein the positions with respect to the optical axis direction as detected by said detecting means and the position with respect to the scan direction at the corresponding moment; and correcting means for correcting, during the scan of the first and second objects, a positional relation between said projection optical system and a surface of the second object to be exposed, with respect to the optical axis direction, on the basis of the stored positional relation between the reference plates and the projection optical system.
The detecting means may include (i) a reflection surface plate providing on said second movable stage at a position substantially corresponding to that of the second object with respect to the optical axis direction, (ii) a light source for projecting, through said projection optical system, a mark of the reference plate onto said reflection surface plate, (iii) light receiving means for receiving light reflected by the reflection surface, through said projection optical system and a transmissive portion of the mark of the reference plate, and (iv) surface position measuring means for measuring a position of one of the second object and said reflection surface plate with respect to the optical axis direction. The position of the reference plate relative to said projection optical system with respect to the optical axis direction may be detected on the basis of an output signal of said light receiving means and the position of said second movable stage with respect to the optical axis direction.
The reference plates may preferably be provided on the first movable stage with deviation.
The angle between said first movable stage and the first object carrying surface thereof may be calculated on the basis of relative positions between the reference plates and the projection optical system, and said correcting means may correct the positional relation between said projection optical system and the surface of the second object to be exposed, continuously on the basis of the calculated angle.
The reference plates are provided at two or more locations, sandwiching the pattern transfer region of the mask upon said first movable stage, in order to enable calculation of an angle between said first movable stage and the first object carrying surface, the angle being defined while taking a line orthogonally intersecting the scan direction on said first movable stage as an axis.
Each reference plate may have marks disposed in a direction perpendicular to the scan direction upon the reference plate, in order to enable calculation of an average angle, taking the line orthogonally intersecting the scan direction on said first movable stage as an axis.
There may be at least two reflection surface plates disposed to sandwich the second object surface, corresponding to scan positions of the reference plate.
The surface position measuring means may comprise off-axis surface position detecting means including (i) a light projecting system for projecting and forming, without said projection optical system, a pattern in an oblique direction with respect to the exposure optical axis onto one of the second object surface and the reflection surface plate, and (ii) a light receiving system for re-imaging an image of the pattern on a surface of a light receiving element, and positional information related to one of the second object surface and the reflection surface plate with respect to the optical axis direction may be produced on the basis of a positional signal of the image of the pattern as re-imaged on the light receiving element.
In accordance with another aspect of the present invention, there is provided a device manufacturing method wherein a first movable stage being movable while carrying a first object thereon and a second movable stage being movable while carrying a second object thereon are scanningly moved in synchronism with each other and relative to a projection optical system so that a pattern of the first object is projected onto the second object, said method comprising the steps of: detecting positions of marks provided on the first movable stage and disposed with a deviation in a scan direction, relative to the projection optical system and with respect to an exposure optical axis direction; measuring a position of the first movable stage with respect to the scan direction; storing the positions with respect to the optical axis direction as detected at said detecting step and the position with respect to the scan direction at the corresponding moment; and correcting, during the scan of the mask and the wafer, a positional relation between the projection optical system and a surface of the wafer to be exposed, with respect to the optical axis direction, on the basis of the stored positional relation between the reference plates and the projection optical system.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.